1. Field of the Invention
The present invention relates to blends of certain copolyesters and a ultraviolet light (UV) absorbing compounds which can be used in connection with sheets, films and other articles as an overlying layer in order to improve the weatherability of an underlying substrate.
2. Background of the Invention
Polymer compositions and particularly polymer compositions suitable for thermoforming have found wide spread acceptance and utility in home, commercial and industrial applications. Many polymeric materials have desirable mechanical and/or optical properties to allow their use for applications such as sheeting and formed parts in particular. Such applications range from outdoor signs, vending machine faces, carport covers, skylights, recreational vehicle covers, such as golf cart tops, off-road vehicle windshields such as for so-called ATV's and snowmobiles and the like, etc. It is well known in the art that polyesters are generally not very stable to the effects of UV radiation. This instability is manifest in a number of ways. The color of the polyester turns yellow, the impact resistance of the material is dramatically decreased and the surface glossiness of the article is diminished or becomes hazed. These problems have long limited the use of polyesters and must be solved in order for polyesters to be fit for articles used in an outdoor environment.
It is also well known in the art that an UV absorbing compound can be incorporated into the polyester article to retard the loss of impact resistance, yellowness development and haze. Generally, the UV absorbing compound has an extinction coefficient much higher than that of the polymer so that almost all of the incident UV radiation is absorbed by the UV absorbing compound rather than the polymer compound itself. The photostabilizers are generally divided into three groups: light screens, ultraviolet absorbers, and quenching compounds. The effects of the latter group, i.e., the quenching compounds, can be explained by the mechanism of energy transfer between the excited polymer molecules and the photostabilizer via dipole-dipole interactions.
Two strategies have evolved for protecting the polyester from the effects of UV radiation: 1) using an acrylic-based polymer layer containing an UV absorber to protect the underlying polyester material, and 2) using a high loading of an UV absorber incorporated directly into a polyester material, which would then be incorporated as a thin layer on the surface of the polyester sheeting material. However, both of these methods are not without their problems.
A serious problem with incorporating a strong UV absorber added to the acrylic layer in order to screen harmful UV radiation from the underlying polymeric material is the differences in rheological characteristics of the polyester substrates and the acrylic material. Another problem is the difference in melt temperatures of the two materials. This usually limits how such constructions are made to the lamination of a film of the acrylic material onto the polymeric substrate during its extrusion which tends to be more costly than a coextrusion process. A serious problem with this methodology is one of eventual delamination, especially in the conditions of high humidity and heat. Delamination, of course, renders the construction useless.
Another problem with using an acrylic protective layer coating is that often the acrylic material is itself very brittle when impacted or bent. Polyesters and copolyesters, while very tough in general, also are known to be generally notch sensitive. That is, when a small notch or scratch is introduced into the surface, the energy required for impact failure is reduced by more than an order of magnitude and the sheeting effectively becomes brittle. As a result of this notch sensitivity, when a brittle acrylic layer is intimately bonded with the underlying polyester or copolyester, a crack initiated in the acrylic layer is quickly and efficiently propagated through the polyester or copolyester substrate. Thus, the entire structure is as brittle as the protective acrylic film even before exposure to weathering.
In the second category, using a polyester layer having an effective level of a ultraviolet light absorbing compound, overcomes many of the problems associated with using an acrylic protective layer. The melt rheologies of the polyester protective layer is very closely matched to that of the underlying material. Additionally the constructions are quite durable and are not brittle. However, this approach too has a deficiency. UV light is able to damage the polymer matrix in which the UV absorber is present, as the UV absorber cannot preferentially absorb 100% of the incident light. That portion of the UV light not absorbed by the UV absorber is absorbed by the polymeric matrix and over long exposure times contributes to damage of the polymeric matrix material as described above, i.e., discoloration, surface damage such as a haze or loss of surface gloss, and property loss, such as resistance to impacts.
U.S. Pat. No. 4,446,262 discloses a photo-stabilized polymer composition having a cyclic imino ester as an ultraviolet absorber.
U.S. Pat. No. 5,108,835 discloses a coextruded thermoplastic sheet having a 5 to 100 micron thick surface layer. The surface layer includes from 0.1 weight % to 20 weight % of an UV absorber. The thermoplastic material includes thermoplastic aromatic polycarbonates, thermoplastic aromatic polyester carbonates, thermoplastic aromatic polyester, thermoplastic aliphatic-aromatic polyesters of iso- and/or terephthalic acid, polyvinyl chloride, polyvinylidene chloride, thermoplastic copolymers of styrene and/or methyl styrene with maleic anhydride and/or with acrylonitrile, thermoplastic graft polymers of styrene and acrylonitrile on polybutadiene rubbers, thermoplastic poly-.alpha.-olefins, such as polyethylene and polypropylene, thermoplastic polyacrylates and polymethacrylates.
U.S. Pat. Nos. 5,558,912 and 5,480,926 disclose a polyethylene terephthalate based copolyester containing 1,4-cyclohexanedimethanol and an ultraviolet absorber selected from cyclic imino esters.
U.S. Pat. No. 5,709,929 discloses a multi-layered plastic sheet having a core section that includes a copolymer prepared by the reaction of dimethyl terephthalate or terephthalic acid with a mixture comprising about 99-55 mole % ethylene glycol and about 1-45 mole % 1,4-cyclohexane dimethanol. The second layer, which has a thickness of between about 2 microns and about 100 microns, overlies one surface of the core section and includes the copolyester of the core section and a UV light-absorbing additive, which preferably includes a benzotriazole or a benzophenone as the light absorbing additive.
DE 2,853,631 A1 discloses that copolyesters of polyethylene terephthalate modified with 1,4-cyclohexanedimethanol are desirable for outdoor use but with a low CHDM content they cannot be adequately stabilized by a benzotriazole compound.
Many UV absorbing compounds have been developed over the years of research in this area. It is also well known that many of these compounds are very specific to the polymer systems they are used in. Certain benzoxazinone compounds are disclosed as ultraviolet absorbers in U.S. Pat. No. 4,446,262. The disclosed benzoxazinones are said to be capable of melt mixing with various polymers. Specifically mentioned thermoplastic polymer classes are polyesters, polyamides, polycarbonates, polyolefins, polyesters and polysulfones. Specifically mentioned polyesters are polyethylene terephthalate, polytetramethylene terephthalate and polyethylene 2,6-naphthalate. These are stated to be optionally copolymerized with trifunctional or higher functional comonomers but no mention of coglycols is made for these materials.
The benzoxazinone compounds disclosed in U.S. Pat. No. 4,446,262 are also disclosed in U.S. Pat. No. 5,251,064 as being useful in a reflecting structure consisting of a metalized polyester layer of the same polyesters as disclosed in U.S. Pat. No. 4,446,262. The preferred polyester in U.S. Pat. No. 5,251,064 is stated to be polyethylene terephthalate.
Other UV absorbers that would result from the substitution of various functional groups onto the aromatic ring(s) of the benzoxazinone system are broadly mentioned in U.S. Pat. No. 4,446,262, and more specifically, in U.S. Pat. No. 5,264,539. In U.S. Pat. No. 5,264,539, for example, an oligomer containing benzoxazinone units is blended with a polyester.
Accordingly, there is still a need for polyester and copolyester blends that are UV light-stabilized suitable for use in outdoor applications.